I've used a meet up.com site here in Milwaukee for the past couple of years with great success. The meetup group already existed and runs a mix of Pathfinder and 4E.

When you post a game only add as many tables as you have confirmed GM's and put in a wait list. I've found that players will volunteer to run another table if needed with little to no prompts on my part. It also helps to request people to put in what scenario they want to play and character level in their RSVP. Not everyone will do it but it helps to see what's got people interested in.

Once you've been going for awhile a level gap will start to show up with the regular players and the new players. Be sure to try and schedule a 1-5 tier for every game day. If new people show up and a table is formed then awesome! If not then the GM gets to play!

Also, personalities will clash. In the end we can't force people to be civil so don't get frustrated.